Imagine sitting in the snow with no coat or jacket, no boots or shoes, no hat or mittens. You would be freezing within minutes! Insects face this same dilemma. They have to find strategies in order to survive the coming cold of winter. In order to survive the extremes of winter, different kinds of insects have evolved to spend the winter in different stages of their life cycle (e.g., crickets as eggs, woolly bear caterpillars as larvae, swallowtail butterflies as pupae, dragonflies as nymphs and honeybees as adults.)
One of the most interesting overwintering strategies is that of the goldenrod gall fly. Goldenrod is a common wildflower found throughout much of Canada, carpeting fields with a warm yellow glow. As fall progresses and the goldenrod’s leaves turn brown and slough off, you may notice small round swellings on its stem. These swellings are the home of the goldenrod gall fly (Eurosta solidaginis) and the start of an amazing story. This is a story that involves feet-tasting, balloon heads and an irritated plant.
Here is how the story unfolds:
- Adult flies, brown and mottled in colour, are only about a ¼ inch in size (5 mm) and emerge from their winter home in late spring.
- As with most insects, adult life is brief—only about two weeks. However, it gives the goldenrod fly just enough time to find
a mate. - The female goldenrod fly selects the plant of her choice by “tasting” the stem with special chemical sensors on her feet.
- She then inserts more than one egg in each stem but only one
egg tends to hatch out for each goldenrod plant. - A tiny white larva hatches and munches its way into the plant stem, creating a hollow chamber. The saliva of the larva irritates the plant, and it responds by growing a round deformation around the chamber. The larva spends the winter, snug in this cozy shelter.
- In the spring, the larva becomes active again and chews out a tunnel as an escape route, almost to the outer surface of the gall. It then moves back towards the center and pupates in a cocoon-like puparium.
- The adult emerges from the puparium, wanders down the tunnel and, by inflating a spiny, balloon-like structure on its head, pushes its way out of the gall. And so, begins another life cycle!
Can you find a gall?
Find a meadow with goldenrods. Look for those round goldenrod galls. If you find one, take a closer look. If you notice a ragged hole in the side of the gall, this means it has been robbed! A chickadee or downy woodpecker has pecked open the gall and eaten the larva inside. When you have found healthy galls, have a parent remove the gall (along with an inch or so of stem) from the plant. You can use scissors or a sharp knife.
Have a parent use the knife to carefully cut into the gall along the stem axis (parallel to the stem), inserting it no more than 1 cm. Twist the knife blade sharply until the gall splits open. Now look for a small white grub (fly larva) inside. You may need to try more than one gall in order to find a healthy grub. Explain once again that the grub will pupate inside the ball in the spring and exit as an adult fly. You can also try putting the grub into a freezer for a couple of hours. When it’s frozen, remove it and watch it warm up and start squirming again. Now, how amazing is that!
Allow each child to take a gall home to witness the rest of the life cycle. Place the gall upright in a glass jar with a lid and place the jar in an unheated location like a balcony or garage. Starting in early May, check the jar for signs of life. With luck, you should eventually find an adult goldenrod gall fly in the jar.
Other goldenrod galls to look for
Other overwintering insects use this same strategy.
Here are some other galls to discover on Goldenrod as winter approaches. Use a magnifying glass or hand lens to get good close look.
- Goldenrod moth gall: Look for these cylindrical (not round) galls on goldenrod. Take a close look at the exit hole. The larva makes a plug to seal off the chamber. It is slightly bevelled, meaning that it is a bit wider along the outside and narrows toward the inside (shaped somewhat like a sink plug). The bevel shape makes it harder for a predator to gain access but easier for the emerging adult moth to pop the plug.
- Goldenrod bunch gall: The tiny goldenrod gall midge (Rhopalomyia solidaginis) lays its egg in a leaf bud of a goldenrod plant. When the egg hatches, the grub somehow stops the stem from growing normally, causing the goldenrod to form a tight, flower-like cluster of leaves at the top of the stalk. While this bunch gall is unique to the goldenrod gall midge, the dense foliage often becomes home to various arthropods, including spiders and other midges. Because of this, the goldenrod gall midge is sometimes called an “ecosystem engineer.”
Other fall insect signs to look for
- Oak apple galls containing larval wasp (apple-like, on oak trees).
- Willow pinecone galls containing larval midge (pinecone-like,
on willows). - Tunnels made by bark beetle larvae in logs or tree trunks.
- Ragged fall webworm nests on numerous broad-leaved tree species.
- Empty tent caterpillar nests on cherry trees.
- Tent caterpillar eggs (inside of shiny, brown rings encircling twigs of cherries).
- European mantis eggs in foamy, tan-coloured cases on twigs and stems.
- Cocoons (pupae) such as the 2-inch (5-cm-) long, brown, silken cocoon of the Cecropia moth.
- Insect nymphs and larvae on or under rocks and branches in streams.
- Hibernating adult insects like mourning cloak butterflies
and queen wasps nestled under loose bark.
